It is widely known that a diffractive optical element having a surface with annular diffractive grating can reduce lens aberration such as field curvature and chromatic aberration (displacement on imaging points due to wavelength). This is because the diffractive grating formed on the diffractive optical element has peculiar properties such as inverse dispersion and anomalous dispersion, and has a high capability for correcting chromatic aberrations. The optical system including a diffractive optical element can reduce the number of optical elements (the number of lenses) by one or two, compared to an optical system of an equivalent capability which does not include a diffractive optical element (for example, an optical system composed only of aspheric lenses). Therefore, the optical system including the diffractive optical element is beneficial for reducing manufacturing cost. Furthermore, the optical system including the diffractive optical element can reduce an optical length. Thus, the optical system can reduce the height as well, which is advantageous for the system. Furthermore, when the optical system includes a diffractive optical element having a blazed diffraction grating or a diffraction grating having a cross-section with small steps internally contacting the blaze, the optical system can make the diffraction efficiency for a single wavelength almost 100%.
More specifically, for example, the depth of diffraction grating with 100% diffraction efficiency of first order diffracted light (thickness of blaze) is theoretically calculated by the following equation (1). Here, λ denotes the wavelength. Furthermore, d denotes the depth of diffraction grating. In addition, n (λ) denotes a refractive index, and is a function of the wavelength λ.
                              [                      Math            .                                                  ⁢            1                    ]                ⁢                                                                                      d        =                  λ                                    n              ⁡                              (                λ                )                                      -            1                                              (        1        )            
As clearly shown in the equation (1), the value of depth d of the diffraction grating with 100% diffraction efficiency changes along with the change in the wavelength λ. More specifically, the diffraction efficiency is not 100% in the wavelength λ different from the specific wavelength (designed wavelength) λ when the value of diffraction grating depth d is fixed. As a result, when a lens used for imaging wide wavelength band (for example, visible light with wavelength approximately from 400 nm to 700 nm) includes a diffractive optical element, unnecessary diffracted light is generated. Subsequently, the generated unnecessary diffracted light appears as flair or a ghost and degrades an image, or reduces the modulation transfer function (MTF) characteristics. More specifically, when the diffraction grating is formed on both surfaces of a single lens or multiple surfaces of the optical system, the unnecessary diffracted light is generated more prominently.
In response to this problem, a diffractive optical element that can reduce the wavelength dependency of the diffraction efficiency has been proposed. FIG. 16 is a diagram illustrating an example of the diffractive optical element 1000 including a protective film 1003 for reducing wavelength dependency of the diffraction efficiency. As shown in FIG. 16, in the diffractive optical element 1000, on the surface where the diffraction grating 1002 is formed, optical material having refractive index and refractive index dispersion different from the base material 1001 is painted or connected as the protective film 1003. The protective film 1003 prevents unnecessary diffracted light from being generated in the diffractive optical element 1000. Furthermore, in such a diffractive optical element 1000, setting, at a predetermined condition, the refractive index of the base material 1001 in which the diffraction grating is formed and the refractive index of the protective film 1003 formed to cover the diffraction grating can further reduce the wavelength dependency of the diffraction efficiency. As described above, the diffractive optical element 1000 including the protective film 1003 indicates, theoretically, high diffraction efficiency in a broad wavelength.
However, even the diffractive optical element with the protective film generates the unnecessary diffracted light because of insufficiently transcribed diffraction grating at the time of forming, or displaced refractive index adjustment of the base material and the protective film material.
Furthermore, even if the diffraction efficiency is high, an absolute luminance value of the unnecessary diffracted light is large when capturing a very brightly shining light source as the object. Thus, an image is formed around the image of object with a saturated luminance value, corresponding to a position where the object is not supposed to exist. In other words, the degraded image quality due to the unnecessary diffracted light becomes a problem particularly when there is a large contrast between the object and the area around the object when a bright object such as a light is captured in a dark background such as nighttime scenery.
In response to this problem, a technique has been proposed for removing an image of unnecessary diffracted light (hereinafter simply referred to as an unnecessary diffracted light image) by calculating the luminance value of the unnecessary diffracted light image, by using least square from a two-dimensional point spread of the unnecessary diffracted light image (for example, see Patent Literature 1).
Furthermore, another technique has been also proposed for removing the unnecessary diffracted light image using two images capturing the same object (for example, see Patent Literature 2). According to the technique disclosed in Patent Literature 2, when there is a pixel with saturated luminance value in the captured image (first frame), the object same as the first frame is captured such that the luminance value of the pixel is not saturated (second frame). Then, the unnecessary diffracted light image included in the image captured in the first frame is removed by calculating the luminance value of the unnecessary diffracted light image using the adjusted value of the exposure time when the second frame is captured.
Furthermore, another technique has been proposed for removing a base color or an offset by detecting a threshold using a histogram indicating frequency distribution of the luminance value (for example, see Patent Literature 3).    [Citation List]    [Patent Literature]    [Patent Literature 1] Japanese Unexamined Patent Application Publication No. 2005-167485    [Patent Literature 2] Japanese Unexamined Patent Application Publication No. 2000-333076    [Patent Literature 3] Japanese Unexamined Patent Application Publication No. 2001-94804